Amanda Fortini's wardrobe was filled with fast fashion. But when she had to grow up quickly, she wanted her clothes to follow suit. The spring collections obliged—gorgeously

Not long ago, while browsing postholiday sales at the mall, I admired a flimsy blue-and-white-striped sequined tank top. My fiancé, an absentminded professorial type whose often disheveled appearance belies his unerring good taste, wrinkled his nose and said, "At this point in your life, you should be buying things of better quality, with more substance and heft." Don't mince words, I thought, and sourly hung the shirt back on the rack. But he was right. The tank top was a very young woman's trifle.

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Although my inner self still feels like she's 16 years old, I'm a journalist in my midthirties. Lately, the universe has been conspiring to send me the message that it might be time to grow up and dress my age. Do I want to be one of those women who refuses to admit that the majority of the insubstantial items worn by girls in their teens and twenties do not flatter someone a decade-plus older? Who hasn't had the trompe l'oeil experience of seeing a teenager in a napkin-size miniskirt or the latest Forever 21 (tight low-rise jeans, say, and a colorful, revealing top) only to be shocked upon glimpsing her from the front that she is middle-aged? The dissonance usually has little do with the woman's physique; rather, it's that such sartorial nothings are at odds with her accumulated life experience, her substantiveness, which tends to show in her face, even if it's altered by Botox and fillers. Her clothes are a small but blatant lie. As Diana Vreeland put it, "The greatest vulgarity is any imitation of youth and beauty."

It's probably no coincidence that my desire to "grow up" my wardrobe comes at the end of a stretch during which I've been forced to grow up myself. A year and a half ago, my fiancé's mother went into a sudden coma and died from a strep infection that had traveled to her brain. For two days, we sat in the dreary concrete hospital in Des Moines, our hands resting lightly on her bare feet, her toenails painted red—even in a pastel hospital gown, with a patch of her head shaved to reveal the vulnerable gray scalp beneath, her innate elegance persisted—as we watched her vital signs rise and fall and rise and fall again. A short time later, an amiable relationship with my ex came to a bitter, litigious, heartbreaking end over a house we own together. None of this made me feel like a naïf, exactly. On a lighter note, for the past few years I've watched my fiancé's daughter, a gangly 10-year-old when I met her, blossom into a lovely, willowy teenager. This has been a delight to witness, but it has also thrown the reality of my own age (not 16) into high relief. I want my clothes to be commensurately adult, to convey some of the gravitas I've earned over time and that I'm increasingly expected to summon in my professional and personal lives.

This urge is echoed by the overarching polish and sophistication of the spring/summer 2013 shows. There wasn't much evidence of the high-low pastiche that we've come to regard as essential to fashion, nor of the fussy ethnic embellishments so prevalent of late. The gestalt was pared down and cleaned up: Designers took a renewed interest in that genteel standby, the suit. Neat tailored jackets, skirts, and trousers were paired with ladylike pointy-toed kitten heels. These were clothes for ladies who want to look like ladies who lunch—e.g., Babe Paley, Jackie O. in her editor years, Kate Middleton in sheer pantyhose and belted coatdresses—even if lunch consists of a confab at a Manhattan power boîte. Think, these days, of Yahoo!'s Marissa Mayer, giving a talk in the gemstone-hue business separates she favors, or director Kathryn Bigelow, chic and sexy in a curve-hugging white pantsuit on a recent cover of Time.

At Miu Miu, Miuccia Prada revisited the '50s with elegant dark denim suits (lined in duchesse satin!) that a modern-day Grace Kelly might wear. At Louis Vuitton, Marc Jacobs went '60s mod, showing numerous iterations of bold checks—and a few splashy flowers—on jackets, tops, and coordinating skirts that grazed thighs, calves, or ankles, but, cheekily, never knees. In his debut for the rechristened Saint Laurent, Hedi Slimane paid homage to the YSL archives of the late '60s and early '70s with bow-tie blouses, cigarette pants, and caftan-style dresses. Playful tweaks kept tasteful classics from being too elderly socialite or mother of the bride: Blazers had boxy cuts and oversize shoulders (Stella McCartney, Chanel), proportions were shrunken, waists nipped, and skirts slit (Marc Jacobs, Calvin Klein Collection, Balenciaga), and there was the occasional wink of kinkiness via leather or lace (Jason Wu, Nina Ricci).

What appeals to me about these clothes is their timelessness, their structure, their impeccable quality. Last winter I got so disproportionately bummed out when a pair of maize-color mittens unraveled after one wearing that I vowed I would never again buy anything else made with chintzy materials or blah design. On a practical level, it's a waste of money and time. But I also think that after you reach a certain age, ephemeral clothing subtly reminds you of all the other impermanence in your life—the relationships that have ended, the places you've lived and had to leave. And such throwaway pieces are nearly always mass-produced. I want clothing that's unique, even rare, clothing that looks like me, the outward expression of my distinctive tastes and triumphs and mistakes, acquired over years. "Life shapes the face you have at 30," Coco Chanel once said. Shouldn't life also shape your wardrobe?

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The truth is, I've always yearned for sophisticated clothes. As a child, I'm told, I begged my mother for "blouses and slacks," probably because I wanted to emulate the all-business, early-'80s attire she adopted as she returned to the workforce. (For Christmas, I requested "office supplies.") A photo shows me, at six years old, behind the wheel of one of those stationary amusement-park rides; I'm dressed in pleated maroon trousers, a white ruffle-front blouse, a skinny leather belt, and huge oval tortoiseshell glasses. I was a mini version of Faye Dunaway in Network, and I'd be right in style this season. Though I eventually gave up the executive-secretary look, I continued to enjoy dressing up, assembling each night before bed my next day's "outfit."

It was only after college, when I wore a salmon-and-beige-houndstooth pantsuit to an interview for an internship at Harper's Bazaar, that my reeducation began. My boss, a stylish woman five years my senior, pulled me aside, told me I "looked like I was interviewing for a job at Senator Moynihan's office," and gave me a tour of Manhattan casualwear—Club Monaco, Zara, Daryl K. I got a lot of use out of the twill pants and cotton tops I bought, since the decade that followed, from the peak of the dot-com bubble in 1999 to the economic crash nearly 10 years later, was a decade of dressing down. Casual Fridays metastasized into casual everyday. The increase in telecommuting meant that workers could (and did) spend the day typing in pajamas, sometimes wearing them out for the quick errand—a slippery slope, to be sure. Fashion blogs celebrating street style began to proliferate; the most photographed practitioners tended to be masterful remixers of down-market finds. During those years, I moved to Los Angeles, a city of feigned nonchalance, where, if you inquire about the dress code for a specific event, people answer, depressingly, "It's L.A." (Translation: "Don't look like you're trying too hard.") Suffice it to say I never mastered the oxymoron of "casual dress." I just got sloppier. So did the rest of America.

But recently, many women have begun dressing up again. Perhaps they're taking a cue from those bloggers, who, upon gaining widespread popularity, became ideal vehicles for product placement—human Christmas trees on which to hang flashy, often elaborate designer ornaments. Some women are pulling together soigné new outfits with the help of a bumper crop of style guides, scouring the stores for the author's must-haves. Or maybe, like me, they've been galvanized by meaningful life experiences of their own. Regardless of her motivation, this season's sophisticated, streamlined pieces make it easy for a woman to channel the voice of her singular style.

Because in the end, of course, what you wear tells a very personal story about you. As young women most of us aren't yet sure what story we want to tell, or we tell many stories at once: I'm responsible enough for the promotion, I'm rebel enough for the bad boy. Cheap, gauzy things require minimal investment of money or self; a change of identity is as easy as a change of your shirt. But with age, the narrative begins to gel. You've made decisions that have defined you. Who you are, what you are—conveying both to the world matters to you. This is why, if you're anything like me, your clothing starts to matter more, too.